Shuffling a cell array (Matlab)

Question

I am currently trying to shuffle the content of a 1 x N cell array in matlab using the follwoing code:

shuffledframes = frames{randperm(NumberOfFrames)};
frames=shuffledframes;

%printing cell array contents
for i=1:NumberOfFrames
    frames(i)
end

However the frame contents do not seem to suffle...

Is there a bug in the code, that i do not see ?

Answer 1

You need to replace

shuffledframes = frames{randperm(NumberOfFrames)};

by either of these:

1. Standard, recommended way:

   shuffledframes = frames(randperm(NumberOfFrames));
   

2. More complicated alternative using lists:

   [frames{:}] = frames{randperm(NumberOfFrames)};
   

Why? In your original code, frames{randperm(NumberOfFrames)} gives a comma-separated list of numbers. Matlab only takes the first number of that list and assigns it to shuffledframes.

In approach 1 above, frames(randperm(NumberOfFrames)) indexes the original cell array with an index vector to produce a new cell array, which is what you want.

Approach 2 has the same desired effect, although it is unnecessarily more complicated. It works by matching one list with another list. Namely, Matlab respectively fills each value of the list frames{:} with each value of the list frames{randperm(NumberOfFrames)}.

To see this more clearly, observe the right-hand side of the first line of your code, and compare with approach 1:

>> frames = {1,2,3,4};
>> NumberOfFrames = 4;
>> frames{randperm(NumberOfFrames)} %// Your code. Gives a list of values.
ans =
     3
ans =
     4
ans =
     2
ans =
     1

>> frames(randperm(NumberOfFrames)) %// Approach 1. Gives cell array.
ans = 
    [3]    [1]    [4]    [2]